1. Field of the Invention
This invention relates to an electrophoretic display device, which performs display by moving charged electrophoretic particles.
2. Related Background Art
In recent years, with advancement of information machinery, the quantity of data of various information becomes larger and larger, and the information is outputted in various forms.
Methods of outputting information are commonly roughly grouped into:
(1) a method of performing display by means of display devices such as a CRT (cathode ray tube) and a liquid crystal display panel; and
(2) a method of performing display by taking hard copies on paper using a printer or the like.
Among the display devices in the method (1), the liquid crystal display panel, is being spotlighted in recent years because of its characteristic features that it requires only a small power consumption and also can be thin-shaped. It, however, also has problems that characters displayed on its screen may be viewed with difficulty depending on angles at which you look at the screen or under the influence of reflected light and that the burden on eyesight which is caused by flickering, low luminance and so forth of its light source has not well been solved.
On the other hand, CRT is superior to the liquid crystal display panel in regard to contrast and luminance. However, in respect of the occurrence of flickering, it can not be said to have a sufficient display quality level compared with the hard-copy display in the method (2). It also has a problem that the display unit is so large and heavy as to have a very low portability.
The hard copies in the method (2) have at first been considered to become unnecessary with spread of display devices, but in fact they are frequently utilized. As reasons therefor, the following can be given. First, it can be pointed out that, in the case of display devices, not only the burden on eyesight has not well been solved as stated above, but also they have a low resolution (120 dpi at maximum), to which the hard copies have a superiority (300 dpi or higher). Second, it can be pointed out that, only the information displayed on the screen can be viewed in the case of display devices, whereas, in the case of the hard copies, a plurality of copies can be arranged to compare each other and can be rearranged without any complicate operation or can be checked in order. Third, it can be pointed out that the hard copies do not require any energy (power) for retaining the display, and are portable as long as the volume (i.e., the number of hard copies) is not so extremely large, making it possible to check the information anytime and anywhere.
As long as any motion picture display or frequent rewriting is not required, the hard copies still have a great advantage compared with display devices as stated above, but on the other hand have a problem that paper is consumed in a large quantity.
Accordingly, in recent years, development is energetically put forward on a rewritable recording medium (a medium on which sharp images can repeatedly be recorded and erased and which does riot require any energy for retaining the display). The third way of display which has succeeded the features of the hard copies and in which the display is rewritable. is herein called xe2x80x9cpaper like displayxe2x80x9d.
Requirements for the paper like display are that the display is rewritable, that any energy is not required or sufficiently a low energy is enough to retain the display (memory performance), that the display has a good portability, that the display has a good quality level, and so forth. An example which, at present, can be regarded as a paper like display is the reversible display medium made from an organic low molecular and high molecular resin matrix system, which which is recorded and erased with a thermal printer head (e.g., Japanese Patent Applications Laid-Open No. 55-154198 and No. 57-82086). Such a medium is some utilized as a display part of a prepaid card, but has problems such that the contrast is not so high and the writing and erasing can only be repeated relatively as small as 150 to 500 times.
As another way of display which is expected to be utilized for the paper-like display, an electrophoretic display device invented by Harold. D. Lees et. al. is known (U.S. Pat. No. 3,612,758). Besides, Japanese Patent Application Laid-Open No. 9-185087 discloses an electrophoretic display device,
The electrophoretic display device of this type has a pair of substrates disposed leaving a gap between them, an insulating liquid with which the gap between the substrates is filled, a large number of colored charged electrophoretic particles dispersed in the insulating liquid, and a pair of electrodes so disposed as to sandwich the insulating liquid between them. In such a device, with changes of the voltage polarity applied to the electrodes, the colored charged electrophoretic particles are attracted to the electrode on this side (viewer""s side) or attracted to the electrode on the other side. When the colored charged electrophoretic particles are kept attracted to the electrode on this side, the color of the particles is perceived. When the colored charged electrophoretic particles are kept attracted to the electrode on the other side, the color of the insulating liquid is perceived. Thus, various images can be displayed by controlling the polarity of applied voltage for each pixel.
In such an electrophoretic display device, however, the colored charged electrophoretic particles are so made as to be freely movable from pixels to pixels, and hence there has been a problem that they can not be distributed in a uniform density, resulting in a poor display quality level.
As a device that can solve such a problem, Japanese Patent Applications Laid-Open No. 59-171930 and No. 01-196094 disclose a display device having a partition wall so disposed as to separate pixels from each other to block any unauthorized movement of colored charged electrophoretic particles.
Now, in the case of the electrophoretic display device of the type described above, a color-developing material (color-developing material such as a dye and ions) must be mixed in the insulating liquid for the purpose of coloring. However, this color-developing material may cause the delivering and receiving of electric charges with the electrophoretic particles, so that it may adversely affect the electrophoretic motion of the electrophoretic particles to lower the performance, lifetime and stability required as display devices.
As a device that can solve such a problem, Japanese Patent Applications Laid-Open No. 49-024695 and No. 11-202804 disclose an electrophoretic display device of a type shown in FIG. 13 (hereinafter xe2x80x9chorizontal movement type electrophoretic display devicexe2x80x9d). Such a horizontal movement type electrophoretic display device has a pair of substrates 1a and 1b disposed leaving a gap between them, an insulating liquid 4 with which the gap between the substrates 1a and 1b is filled, a large number of colored charged electrophoretic particles 5 dispersed in the insulating liquid, and a pair of electrodes 66 and 67 disposed in each pixel A. The pair of electrodes 66 and 67, however, are not so disposed as to sandwich the insulating liquid 4 between them like the type described previously, but are so disposed as to be arranged on one substrate 1a. In the case of such a horizontal movement type electrophoretic display device, the insulating liquid 4 may be transparent and any color developing material need not be mixed. Hence, the problem as stated above can be avoided. Also, in this device, one electrode 66 (here a first electrode 66) is covered with a colored layer 8a having the same color (e.g., black color) as the charged electrophoretic particles 5, and the other electrode 67 (here a second electrode 67) is covered with a colored layer 8b having a different color (e.g., white color). The colored charged electrophoretic particles 5 move horizontally (in the direction parallel to the substrate) in accordance with the polarity of the voltage applied to the electrodes 66 and 67, and are attracted to the first electrode 66 or second electrode 67. When the colored charged electrophoretic particles are kept attracted to the first electrode 66, the color of the particles is readily perceived. When the colored charged electrophoretic particles 5 are kept attracted to the second electrode 67, the whole pixel is perceived in the same color as the charged electrophoretic particles 5. Thus, various images can be displayed by controlling the polarity of applied voltage for each pixel.
Now, in the horizontal movement type electrophoretic display device as described above, colored charged electrophoretic particles 5A1 disposed in a certain pixel A1 are desired to be controlled only by the voltage applied to electrodes 66A1 and 67A1 of the pixel A1. However, the colored charged electrophoretic particles 5A1 may move irregularly under the influence of an electric field of an adjoining pixel A2 (i.e., the voltage applied to electrodes 66A2 and 67A2 of the pixel A2) to cause a disorder of display and a decrease in contrast.
In the case of a method of performing display by forming electric field distribution in the horizontal direction (i.e., in the above horizontal movement type electrophoretic display device), like electrophoretic display intended in the present invention, various wirings and switching elements connected to electrodes and switching elements may partly stand uncovered in regions where any electrodes (display electrodes) are formed (e.g., the boundary region between the first electrode and the second electrode and the boundary region between pixels), and there has also been a serious problem that any leaked electric field the wirings and switching elements may cause makes the electrophoretic particles move in the vicinity of the wiring of display pixels, resulting in a poor display quality.
Accordingly, an object of the present invention is to provide an electrophoretic display device which prevents any deterioration of display quality stated above.
The present invention was made taking account of the above circumstances, and provides an electrophoretic display device, comprising:
a pair of substrates opposing to each other with a gap therebetween,
a spacer keeping the gap between the substrates to a predetermined extent,
first electrodes and second electrodes disposed on either one of the substrates,
an insulating liquid filling the gap between the substrates, and
a plurality of charged electrophoretic particles carried in the insulating liquid,
the plurality of charged electrophoretic particles being capable to move in response to a voltage applied between the first and second electrodes, wherein,
the display region of the device is divided into pixels each having at least a pair of the first and the second electrodes, at least one of the first electrodes in a pixel is disposed along and adjacent to the whole or a part of the boundary of the pixel, and
the one of the first electrodes in a pixel and said one of the first electrodes in another pixel which are disposed adjacently to each other via the pixel, boundary have the same potential.